Semiconductor devices are very small, typically from 5 mm×5 mm square to 50 mm×50 mm square, and typically comprise numerous sites for the bonding of electrical conductors to a semiconductor substrate. Each bond consists of a solder ball/bump or copper pillar or wire.
It is necessary to test the bond strength of the bonds, in order to be confident that a particular bonding method is adequate. Because of the very small size of the bonds, tools used to test the bond strength of these bonds must be able to measure very small forces and deflections accurately.
There are several different types of bond tests that are used to test bond strength. For example, shear testing tests the shear strength of a bond by applying a shear force to the side of the bond and shearing the bond off the substrate. Pull testing tests the pull strength of the bond by pulling the bond away from the substrate. In a push test, a force, or load, is applied in the vertical plane directly downward onto a bond.
Machines that perform these tests typically comprise a bond test tool, be it a shear test tool, push test tool or a pull test tool, that can be positioned relative to the bond under test and then either the bond or the tool are moved in order to perform the test, which comprises measuring the force needed to break the bond. Each bond test tool is mounted to a load cell provided with a force transducer and associated electronics.
While there are a variety of different test tools, the part of the machine used to position the test tool and move the test tool or bond during a test may be the same for each test tool. Accordingly, test tools have been designed to be removable from the rest of the machine so that they can be replaced with a different type of test tool or a test tool suitable for a different range of measurements or a different type of bond or test. It may be desirable to use several different test tools on a single substrate under test and so an operator may be required to perform replacement of test tools frequently.
The requirement to be able to measure small forces accurately leads to certain requirements for the performance of each test tool. Shear test tools are used to shear a bond off a substrate and the force required to shear the bond is recorded. A force transducer is coupled to the shear test tool in order to measure the force. In order to ensure repeatability for shear measurements, it is essential for the tip of the test tool to engage the side of the bond at a predetermined height above the surface of the substrate. This distance is small but critical, since the bond is usually dome-shaped. A predetermined spacing from the surface eliminates both sliding friction from the test tool on the substrate, and ensures that the shear load is applied at a precise location in relation to the bond interface. Accordingly, in practice, the test tool is first moved into contact with the substrate surface and then withdrawn by a predetermined distance, typically 0.05 mm or less before the shear test is performed.
Several difficulties arise. Friction and stiction in the mechanism of the device itself may cause difficulties in sensing contact with the substrate surface. Imprecise surface sensing will inevitably affect the distance by which the test tool is withdrawn, and thus the height at which the bond is sheared. The distances involved are very small and so care needs to be taken to sense the exact moment of surface contact, without compression of the substrate. Care must also be taken to prevent uncontrolled movement of the test tool at the test height prior to or during the shear test. Such movement may seriously affect the accuracy of the test results and significant movement of the test tool at the test height may damage an adjacent bond or wire.
The twin objectives of both a low contact force when sensing the surface of the substrate and accurate control of the test height are difficult to resolve.
U.S. Pat. No. 6,078,387 discloses a device for sensing contact of a test head of a test tool with the substrate which is adapted to immediately stop downward drive of the test head when contact is sensed. The test tool is supported on the free end of a pair of parallel flexures, which are secured at their opposite ends to a base plate and deflect to allow some vertical movement of the test head with respect to the base plate. To prevent vertical movement of the test head during the shear tests, the test tool is spring biased by the flexures against the base plate. The test head can be moved away from the base plate by an air-bearing to allow the test head to move vertically in a substantially frictionless manner for initial contact sensing. Thus, when the test head first touches the substrate surface, it is pushed back by the substrate surface on the flexures. Movement of the test head relative to the base plate or movement of the flexure can be detected by an optical detector, and the air-bearing is then switched off to ensure that the test head is fixed relative to the base plate by the spring bias of the flexures against the base plate. Once the test head is fixed relative to the base plate, the base plate is raised by a predetermined amount so as to leave a clearance between the lower end of the test tool and the substrate of the desired “step off distance”.
An alternative system is to have the parallel flexures bias the test tool away from the base plate to allow for substantially friction-free movement of the test head relative to the base plate during initial positioning, but then to press the test tool into contact with the base plate using a piston driven by compressed air to create a clamping force on the test tool against the base plate during a test procedure. A further alternative, described in EP2363701A, is to have the flexures bias the test tool away from the base plate to allow for substantially friction-free movement of the test head relative to the base plate during initial positioning, but then to clamp the test tool against an abutment fixed to the base plate using a piston driven by compressed air. The abutment may be complemented by a further abutment positioned to provide an additional clamping force during a shear test.
In all these alternatives, the use of flexures provides the ability to sense the surface of the substrate without damaging the substrate. The test tool is then prevented from movement relative to the base plate by a pneumatically operated clamp.
Pull and push testing typically use hook or jaw tools and requires a different configuration for the load cell. However, the tools may also employ a pair of parallel flexures, with one or more strain gauges positioned on the flexures to detect the force applied in a vertical direction. The flexures must be highly compliant in order to control the loading rate on the bond and to minimise damage due to accidental contact with the substrate. An example of this type of pull testing tool is described in U.S. Pat. No. 6,301,971B1.
While the machines described offer excellent measurement of the force needed to break a bond, it would be desirable to improve the ease of use of the machines. In particular, it would be desirable to improve the ability to switch between different test tools while retaining a high accuracy of measurement. It would also be desirable to be able to make such an improvement possible for existing machines.